1. Field of the Invention
The present invention relates to an objective lens, an optical pickup device for carrying out at least one of the record of information on a plurality of optical information storing media in which the transparent substrates have different thicknesses from each other and the reproduction of information from the optical information storing media by using a condensing optical system including the objective lens, a reproducer and a recorder.
2. Description of Related Art
In recent years, when a short wavelength red laser is practically used, a DVD which is a high density optical information storing medium (also referred to as “optical disk”) having the same size as a CD (compact disk) and a high capacity is commercialized. In a recording/reproducing apparatus for DVD, a numerical aperture on an optical disk side of the objective lens is 0.6 to 0.65 when a semiconductor laser emitting a light having a wavelength of about 650 nm is used. The DVD has a track pitch of 0.74 μm and a shortest pit length of 0.4 μm. The DVD has a 4 or more times higher density than the CD having a track pitch of 1.6 μm and a shortest pit length of 0.83 μm. In the DVD, in order to suppress a comma aberration caused by inclining the optical disk to an optical axis, the thickness of the transparent substrate is 0.6 mm and is a half of the thickness of the transparent substrate of the CD (1.2 mm).
In addition to the CD and the DVD, optical disks having various standards in which the wavelengths of lights emitted from the light sources, the thicknesses of the transparent substrates and the like are different from each other, for example, CD-R, CD-RW (write once type of compact disk), VD (video disk), MD (mini disk), MO (magneto optical disk) and the like, are commercialized and popularized. Further, the semiconductor laser emitting a light having a shorter wavelength has been developed. A short wavelength blue light laser having an oscillation wavelength λ of about 400 nm will be practically used. Because the wavelength becomes short, it is possible that the optical information storing medium has a higher capacity even though an objective lens having the same numerical aperture as the objective lens for DVD is used.
Further, a plurality of optical information storing media having the same size as CD which is the above-described former optical information storing medium, for example, CD-R enabling the record and the reproduction, DVD enhancing the recording density and the like, in which the thickesses of the transparent substrates of the recording layers and the wavelengths of the laser lights for record and reproduction are different from each other, have been developed. For these optical information storing media, it is required to enable the record and the reproduction by using one optical pickup device. Therefore, each type of optical pickup device having a plurality of laser light sources corresponding to each working wavelength and condensing a light on a recording layer by using the same objective lens with each necessary numerical aperture is proposed (for example, Japanese Patent Application Publications No. Tokukai-hei 9-54973, No. Tokukai-hei 11-96585 and No. Tokukai-hei 11-86319 and the like).
Among them, in Japanese Patent Application Publication No. Tokukai-hei 9-54973, an optical system using a hologram optical element which utilizes a transmitted light having a wavelength of 635 nm (0th order diffracted light) and a −1st order diffracted light having a wavelength of 785 nm, and an optical system using a hologram optical element which utilizes a +1st order diffracted light having a wavelength of 635 nm and a transmitted light having a wavelength of 785 nm (0th order diffracted light) are disclosed. However, according to those hologram optical elements, because a difference between adjacent steps of the hologram is large, it is difficult to unite them with an objective lens.
In Japanese Patent Application Publication No. Tokukai-hei 11-96585, an objective lens in which three divided surfaces are provided on a refractive surface of the light source side, is disclosed. Further, an optical pickup device which utilizes a light flux passing through the first divided surface and the third divided surface when information is reproduced from the first optical disk and utilizes a light flux passing through the first divided surface and the second divided surface when information is reproduced from the second optical disk in which the thickness of the transparent substrate is different from that of the first optical disk, is disclosed. However, according to this objective lens, when information is reproduced from the optical disk having a thicker transparent substrate, for example, CD, a residual aberration becomes larger.
In United States Patent Application Publication No. 2001-8513 which was assigned to the same assignee as this application, an optical pickup device in which a spherical aberration is compensated for a plurality of light sources having different oscillation wavelengths from each other, by canceling the function of a diffractive surface and that of a refractive surface with the objective lens having a diffractive ring-shaped zone on the refractive surface, was proposed. In this case, when a wavelength of a light emitted from the light source having a shorter oscillation wavelength is changed, there is some possibility that the chromatic aberration is caused.